What is the reason for a blunt soreness in the area right under my jaw in the back. It might be my submandibular gland, what causes that sort of pain?

Infection, stones. Hello, the submandibular gland can swell and become tender from infection or small stones that can form in the ducts of the glands. You can try massage, increased fluids and lemon drops. The soreness could also be a swollen tender lymph node. I encourage you to be checked if it doesn't resolve within a few days or if you get acutely worse.
2 possibilities. Pain under the jaw may indeed be inflammation of the sub-mandibular gland. The pain is from inflammation, the body's attempt to ward off the spread of an infection. The cause may be a stone in the duct of the gland which can be removed by a specialist. One other possibility is the soreness may arise from spasm of the internal pterygoid muscle just behind the corner of the upper jaw.